The invention relates to optical modulators and switches which utilize closed loop resonators.
Optical communications has become the prevailing technology for long haul communications, and is now making tremendous inroads into local communications networks, local area loops, and distributed computing. The domination of optics has transpired due to the higher bit rates, lower distortion, and lower loss possible using fiber optics compared with that using any form of electrical transmission. However, information transducers such as telephones, video cameras, computers, and the like have their information content originating in electronic format. These electrical signals must be converted to optical modulation before being transmitted. In addition, once in optical form, these signals must be switched and routed through the optical network to arrive at their final destinations. Thus, devices which perform this electro-optical conversion and optical routing are fundamentally required in any optical communications system.
FIGS. 1A and 1B are schematic diagrams of the basic principles of modulators and switches, respectively. Optical modulators, such as optical modulator 100, are the devices which perform the task of impressing onto a lightwave signal, information carried by an electrical signal. Optical switches or routers on the other hand, such as optical switch 110, are the devices which provide a means of diverting an input optical signal to one of a number of possible output ports, thus redirecting a signal according to its intended destination. Some devices can be used in both capacities. Modulators and switches are controlled by an electrical signal. In the case of the modulator 100, a waveform 102 of the electrical signal is to be reproduced on the optical input signal 104 to produce an optical output signal 106. In the routing switch 110, the electrical signal 112 is used as a control line, which establishes the creation of the desired path between input 114 and output 116 ports.
There are four widely accepted performance criteria by which the merits of modulators and switches may be standardized. They are as follows: (1) Modulation Depth, or ON/OFF contrast, which is the ratio of minimum to maximum swing in optical power achievable; (2) Cross-talk, which is the fraction of input power unintentionally coupled to a specific output port, when that port is not selected by the switch; (3) Modulation Bandwidth, which is related to the maximum modulation frequency at which the device may operate (typically the bandwidth is limited by the electrical layer due to electrode capacitance, but may be also limited by the wavelength selectivity of the optical layer); and (4) Electrical power consumption, which involves both the power to switch and the power to hold a given (ON/OFF) state.
Four of the more promising modulator and switching schemes proposed and demonstrated to date are shown in FIGS. 2A-2D. Those devices which modulate based on an induced index change are classified as electrooptic modulators, while those which make use of absorption are called electro-absorption modulators. The simplest scheme makes use of directly modulating a laser as shown by the modulator 200 in FIG. 2A, which is a form of electro-absorption modulation. FIG. 2B shows a Mach-Zehnder interferometer modulator 202. FIG. 2C shows an acoustooptic modulator 204 which can also serve as an optical switch. FIG. 2D shows a basic electro-absorption modulator 206.
The primary limitation of all the preceding modulation devices is their relatively long lengths. This is especially true of electrooptic modulators, (FIGS. 2B and 2C), as the induced index change is small, and significant effects are only accumulated over long propagation distances. The associated long electrodes create large capacitance which limits high frequency modulation.
Furthermore, in communications systems a switching substation may require hundreds to thousands of switches, all of which are interconnected. The large size of currently proposed switches means that at most a few devices can be interconnected on a single chip. The remaining majority will have to be interconnected as discrete components. This increases the cost, size, and power consumption, and decreases the performance to such an extent that the benefits of optical switching may no longer outperform electrical domain switching. Electro-absorption modulators can be shorter and more efficient than electrooptic modulators. However, all electro-absorption modulators proposed to date (which are variants of FIG. 2D), absorb the optical power, and therefore can not be used as switching elements.
It is therefore an object of the invention to provide a novel device which can make use of absorption to switch or modulate an optical signal, without incurring significant signal attenuation. It is also an object of the invention to utilize a resonator, which recirculates the optical power, thus making the device size several orders smaller than those proposed to date.